This invention relates to control systems for electric motor driven cyclic loads and more particularly relates to a control system for an oil well beam pumping unit.
Crude oil occurs in oil bearing strata which may be many thousands of feet below the earth's surface. To produce this oil, wells are drilled and the fluid that collects in them is lifted to the surface, often by some means of artificial lift. The most common lifting device is a beam pumping unit, where a string of steel rods is hung from the beam pumping unit at the surface through the well down to a reciprocating pump at the oil strata level. The beam pumping unit at the surface imparts an up and down motion to the rods which in turn reciprocate the downhole pump to lift the fluid.
A typical beam pumping unit consists of the following components in power train order: an electric motor, belts and sheaves, gear box, crank and counterweights, pitman, walking beam, rod string, and downhole pump. The walking beam is moved up and down by a pivoted linkage (the pitman) to the rotating crank. The crank is rotated by the motor via the belts and sheaves and gear box. One complete crank rotation reciprocates the pump through one complete cycle of one upstroke and one downstroke.
The reciprocating action of the downhole pump imposes an intermittent load on the rod string. The fluid must be lifted on the upstroke, but not on the downstroke. The counterweights are placed at one end of the crank and are sized and phased to halve the load of the fluid and to double the loading frequency, i.e., the counterweights halve the effective load on the up stroke and provide artificial load on the downstroke. Nevertheless, the loading required to operate the beam pumping unit still varies dramatically throughout any one cycle of the pumping unit and is effectively reduced to zero as the downhole pump passes over the top of an upstroke and the bottom of a downstroke. This pumping load is seen as a widely varying speed and torque requirement by the electric motor.
An electric induction motor is normally a cost effective way of converting electrical energy to mechanical power, yet it is not suited to a varying load. To accommodate the varying load of the beam pumping unit, it is typical to choose a high slip version (Nema D) of electric induction motor in order to allow a small crank speed variation, even though there is an inherent loss of efficiency in doing so.
When an electric induction motor is used to power a beam pumping unit, energy losses in the surface equipment are even higher than expected. Although this problem has long been recognized, it is only in recent years that it has become important to the operator of beam pumping units. This is because with the increasing price of electricity and the decreasing percentage of oil produced per unit of electricity, electricity costs have become a large part of total oil production costs.
The basic cause of the problem is the mismatch between the power source and the load. An electric motor is designed to output a fairly constant level of mechanical energy, but the beam pumping unit is an intermittent or cyclic load which requires widely fluctuating power at the crank shaft to turn the crank through one cycle.
In addition to the lack of concern over energy losses in beam pumping units until recent years and therefore its nonrecognition as a serious problem, the nature of the energy loss problem in beam pumping units has been obscured by at least three factors:
(1) The detailed performance of electric induction motors under widely varying load conditions is not generally well understood, particularly when the motor behaves as a generator. PA1 (2) The analysis of the behavior of the rod string, to which the surface equipment is attached, is particularly complex and requires the iterative use of mathematical algorithms. These algorithms are best performed by computers which inhibit further engineering insight. PA1 (3) The analysis of the surface equipment performance of a beam pumping unit has traditionally been neglected because it does not significantly affect the choice of system components. In the past, the important parameters governing choice of equipment have been gear box torque and polished rod load.
Therefore, only parts of the problem have been correctly perceived and only partial solutions have been attempted. In fact, the issue of surface efficiency of a beam pumping unit has not often been addressed directly.
For example, past efforts to improve efficiency have focused on the perceived problem as being the large difference between the peak and average torque required by the beam pumping unit. The solution was to attempt to average or smooth out the mechanic load by changing the geometrical arrangement of the articulating and rotating subcomponents of the beam pumping unit and by improving the strength to weight ratio of the rod material. Both of these approaches reduce energy losses, although not normally by a very large amount.
Other efforts have perceived the problem as being that the motor is overloaded and underloaded through one cycle of the beam pumping unit with big differences between the peak torque and average torque required. The solution was to use an ultra-high slip motor to allow large speed variation in the motor. The ultra-high slip motor has a smaller than normal variation of torque output as its speed is varied, and thus the motor allows the speed to fluctuate as the load torque varies. The torque created by the acceleration and deceleration of the rotating components therefore reduces the peak and minimum torque seen by the gear box and the motor. In this way, gear box stresses and motor overload and underload are reduced. The ultra-high slip motor achieves this at the expense of low motor efficiency. Further, this approach cannot be taken to the logical extreme because very large motors are then needed and the motor becomes even more inefficient.
Other efforts have perceived the problem as being that speed variation in the cyclic load causes overload and underload of a standard electric motor and thus exacerbates energy losses. The solution was to attempt to hold the motor speed fairly constant, using a flywheel alone. This was attempted with a Nema D motor, therefore the size of the flywheel required was very large and the efficiency improvement was not very great.
Other efforts perceive the problem as being motor inefficiency due to varying loads and have attempted to solve the problem using variable frequency power supplies alone to avoid peak loads. This approach has been attempted several times unsuccessfully.
As previously mentioned, these prior attempts have only correctly perceived parts of the problem and have therefore only applied partial solutions.
The present invention identifies the cause of the extra and unexpected energy losses as the process of regeneration within the beam pumping unit. The problem is not the regenerated energy itself, which is not lost, but losses inherent in the act creating and transferring the energy to provide regeneration.
During a single crank cycle or pump stroke (one complete upstroke and downstroke), of the beam pumping unit, the crank turns one complete revolution. Normally, there are considerable periods during this revolution or cycle when the beam pumping unit actually forces the motor rotor to speed up (negative torque load). Under these circumstances, the motor rotor speed is often forced above the synchronous speed (defined by the speed of rotation of the magnetic field in the stator) and the motor becomes a line excited generator which feeds power back to the electrical supply while at the same time acting as a brake on the mechanical parts of the beam pumping unit, i.e., resisting attempts of the rotating crank and counterweights to speed up the rotor. This phenomenon is known as regeneration and is responsible for the severe increase in energy losses, both in the motor and in the beam pumping unit components.
It is important to distinguish between the recoverable energy, which is the regenerated energy returned to the power source, and the unrecoverable energy losses which are incurred in the various system components by the act of regeneration. It is not the regenerated energy itself that causes the inefficency problem, but instead it is the losses incurred in the process of producing the regenerated energy. This process consists of drawing the extra energy (which is to be regenerated) from the power source, storing it in the beam pumping unit, and in returning it to the line as regenerated energy. Obviously, this process cannot be accomplished loss free and therefore creates the unrecoverable energy losses and inefficiencies remedied by the present invention.